Glycolysis is the main pathway for carbohydrate catabolism. It is a process in which monosaccharides are broken down to pyruvic acid, two molecules of which are formed per monosaccharide residue. In plants D-glucose and D-fructose are the main monosaccharides catabolized by glycolysis although other monosaccharides that can be converted to glucose or fructose can be handled by this catabolic pathway. In cells where photosynthesis is not taking place glycolysis is a key metabolic component of the respiratory process which generates energy in the form of ATP. Typically the cells of germinating seedlings and non-photosynthetic cells of mature plants utilize this metabolic pathway. The glycolytic pathway is controlled in part by the potent allosteric regulator fructose-2,6-bisphosphate (F2,6P). This regulatory molecule activates the enzymatic activity of phosphofructosekinase (PFK) which stimulates the flow of carbon through the glycolytic pathway to pyruvate. PFK plays a central role in the control of glycolysis because it catalyzes one of the pathway's rate-determining reactions. F2,6P also inhibits the activity of fructose bisphosphatase (FBPase) which stimulates the flow of carbon through gluconeogenesis, to form glucose. The concentration of F2,6P in the cell depends on the action of 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase (PFK-2/FBPase). The formation and degradation of F2,6P is catalyzed by PFK-2 and FBPase-2, two enzyme activities that occur on different domains of the same protein molecule (Algaier, J. et al. (1988) Biochem Biophys Res Commun 153(1):328-333). Thus, 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase is a key regulatory enzyme that controls carbon flux through glycolysis vs. gluconeogenesis. Because PFK-2/FBPase regulates the abundance of a key allosteric regulator, manipulating the activity of this enzyme either by controlling expression or by directed mutagenesis could be used to control carbon flux through the glycolytic of gluconeogenic pathways. This could be very important in bioprocessing in plants.
Respiration (aerobic metabolism) takes place in the mitochondria in most eukaryotes. The ubiquinol-cytochrome C reductase (bc1) complex is an important component of the mitochondrial electron transport system. The BCS1 gene encodes a product that has been shown to be necessary for the expression of the Rieske iron-sulfur protein a component of the bc1 complex (Nobrega, F. G. et al. (1992) EMBO 11:3821-3829). By controlling the expression of BCS1 it may be possible to modulate the level of the Rieske iron-sulfur protein in plant cells which it turn would regulate the amount of functional ubiquinol-cytochrome C reductase complexes in mitochondria.
Few of the genes encoding the 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase and BCS1 proteins in corn, Momordica, rice and wheat, have been isolated and sequenced. For example, no corn, Momordica, rice or wheat genes have been reported for 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase and no plant genes have been reported for BCS1. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these proteins would facilitate studies to better understand carbon flux and respiration, provide genetic tools for the manipulation of these metabolic pathways, and provide a means to control glycolysis and respiration in plant cells.